Hypertension is a major risk factor for cardiovascular and cerebrovascular disease, and patients with mild hypertension account for the majority of the morbidity. The treatment of these patients is controversial because the results of drug therapy have been disappointing. Dietary sodium restriction and calcium supplementation are interventions that lower blood pressure in some hypertensive patients. The objective of this application is to determine the effectiveness and feasibility of sodium restriction and calcium supplementation in a community based hypertensive population, to determine the interrelationship of these two ions, and to test the ability of several markers to predict the response to these interventions. The markers to be investigated are the activity and reactivity of the renin-angiotensin system, indices of calcium and magnesium metabolism, and atrial natriuretic factor. A random sample (250 patients) will be selected from a large population of hypertensive patients from a Hypertension at the Worksite Treatment Program. A baseline evaluation will vb followed by four week periods of high and low sodium intake, each of which will be preceded by a four week periods of high and low sodium intake, each of which will be preceded by a four week period on a medium sodium intake. The order of high and low sodium diets will be randomly assigned. The same subjects will then enter a placebo controlled crossover study of the effects of oral calcium supplementation, each phase lasting three months. The effects on blood pressure will be assessed by measurement at the worksite, home blood pressure recordings, and 24 hour ambulatory blood pressure monitoring. Twenty-four hour blood pressure monitoring will allow comparison of the effects of the changes in calcium and sodium intake on work and home pressures, providing an index of their effects on blood pressure reactivity. The relationship of sodium sensitivity to calcium sensitivity will be assessed. The ability of baseline levels of plasma renin activity, renin response to acute converting enzyme inhibition, baseline levels of serum ionized calcium and magnesium, and baseline levels of atrial natriuretic factor to predict the response to manipulation of sodium and calcium intakes will be assessed, as will the effect of the dietary manipulations on these parameters. Results of this should have the capability for providing to health care workers new, practical and safe means for blood pressure control in large populations.